FIG. 1 is a block diagram of a conventional typical encoding apparatus for encoding a moving picture signal.
The encoding apparatus shown in FIG. 1 includes a local decoding device and is provided with frequency transformation unit 101, quantization unit 102, variable-length encoding unit 103, inverse quantization unit 104, inverse frequency transformation unit 105, frame memory 106, motion compensation unit 107, motion estimation unit 108, and code amount control unit 109. The encoding apparatus is also provided with subtracter 121, switch 122, and adder 123.
An input picture frame is supplied to the encoding apparatus and is divided into a plurality of blocks. When inter-frame prediction is used, prediction values described later are subtracted from the divided blocks by subtracter 121.
The inter-frame prediction is a method for predicting a current frame using a picture frame which is previously reconfigured. An input block from which the prediction value is subtracted is called a prediction error. It should be noted that a picture frame in which all blocks in an encoding frame are encoded only by intra-frame prediction for producing prediction values from adjacent pixels in the same encoding frame is called I picture. A picture frame in which all blocks are encoded by using the intra-frame prediction and the inter-frame prediction is called P picture. Also, in the inter-frame prediction, a picture frame encoded by referring to a plurality of reconfigured frames inputted previously and subsequently to the current encoding frame is called B picture.
In general, in encoded moving picture data, I pictures are set in a constant cycle, and a section including a plurality of frames divided by the I picture is call a GOP (Group Of Picture). These definitions of the I, P, and B pictures and the GOP are used in MPEG (Motion Picture Expert Group) scheme which is a moving picture encoding standard of the international standardization.
Then, the input block in which the prediction value is subtracted is transformed into a frequency domain by frequency transformation unit 101. The input block transformed into the frequency domain is quantized by quantization unit 102. The quantized input block, namely, a transform coefficient is entropy-encoded by variable-length encoding unit 103 and outputted. Switch 122 is provided to switch whether the inter-frame prediction is performed or not. When the inter-frame prediction is not performed, through switch 122, an input block in which no prediction value is subtracted is directly given to frequency transformation unit 101.
A quantized prediction error is returned to a prediction error in an original spatial domain by inverse quantization unit 104 and inverse frequency transformation unit 105 as a local decoding process. Further, the prediction value is added to the prediction error returned to the spatial domain by adder 123, and is stored in frame memory 106 as a reconfigured picture.
The reconfigured picture stored in frame memory 106 is referred by motion compensation unit 107 and motion estimation unit 108 in order to produce a prediction value. Accordingly, the reconfigured picture stored in frame memory 106 is also called a reference frame.
Motion compensation unit 107 produces the prediction value from the reference frame stored in frame memory 106 using a motion vector and a reference frame index supplied from motion estimation unit 108. Motion estimation unit 108 detects the motion vector of the input block and the reference frame index indicating a frame number to be referred so as to make a difference between the input block and the prediction value, namely, the prediction error smallest.
A bit stream which is moving picture information compressed by the above-mentioned process is a variable-length code mainly including a transform coefficient, a quantization parameter, a motion vector, and a reference frame index of each block.
Now, generally, in a digital broadcasting system, a picture communication service, or the like, a generated code amount of moving picture signals, namely, a bit rate is controlled for transmission and storage. So, the code amount control unit 109 monitors the generated code amount supplied from the variable-length encoding unit 103, and makes the quantization parameter larger to decrease the transform coefficient when the generated code amount is likely to exceed a target bit rate, and makes the quantization parameter smaller to increase the transform coefficient when the generated code amount is likely to be below the target bit rate. With this operation, the generated code amount is controlled in the conventional moving picture encoding technique.
Usually, in order to encode a fixed information amount of moving pictures of high picture quality, it is more preferable to increase a ratio of an information amount of the transform coefficient included in the bit stream. As its reasons, the transform coefficient is just information representing the texture of the picture.